So far, the compounds of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, yttrium, scandium, nickel, cobalt, iron, aluminium, gallium, indium, manganese, cadmium, zirconium, hafnium, strontium, barium, copper and zinc etc are the most common inorganic compounds in industry. They are widely used in the field of metallurgy, machinery, petroleum chemical industry, light industry, electronic information, energy, environmental protection, defence military industry, new-high tech materials etc. They are the crucial and indispensable raw materials for developing new-high technology and top national defense technology, and also reforming traditional industry all over the world. Therefore, they become the important strategic resources in new technology revolution and international competition.
The metal resources in the world are abundant, and new resources are discovered continuously. Generally, metal concentrate is firstly decomposed and leached by acid or alkali etc to obtain mixed-metal-ions solution of chloride, sulfate or nitrate etc. And then purification and separation methods, such as solvent extraction, redox method, ion exchange or extraction-chromatography method etc, are used to obtain single metal solution, for example, single metal chloride, single metal sulfate, or single metal nitrate etc. Next, the mentioned single metal solution is generally precipitated using oxalic acid, and then calcined to produce metal oxide. The product obtained from this method possesses high purity. However, as oxalic acid is expensive, the production cost is high, and moreover, a lot of oxalic acid wastewater is produced. Some product is obtained as metal carbonate by using ammonium bicarbonate as precipitant, and also metal oxide is obtained by calcination. Or if ammonium hydroxide is used as precipitant, the metal hydroxide is obtained. The cost of using ammonium bicarbonate or hydroxide as precipitant declines significantly, but at the same time a lot of ammonium-nitrogen wastewater is produced leading to environment pollution. The disposal cost of ammonium-nitrogen wastewater is very high, and it is difficult to meet the emission standard. In order to avoid the pollution of ammonium-nitrogen wastewater, some companies use sodium carbonate or sodium hydroxide instead of ammonium bicarbonate or ammonium hydroxide as precipitant to produce metal carbonate, metal hydroxide or metal oxide. However, the cost increases about one time, and the pollution of sodium salt wastewater occurs.